Honeycomb core is commonly used in the fabrication of panels and other assemblies in the aerospace industry as the core provides structures with high strength and low weight. Honeycomb core panels are comprised of a honeycomb core that is bonded between face sheets. Acoustic honeycomb panels modify a structural assembly by forming resonant cavities within the core by perforating face sheets of the panels and/or fabricating internal septum membranes within their lumen for modifying sound.
Engine housing nacelle panels, as a specific example, are made of a honeycomb core that can be modified with porous internal and/or face sheets that create Helmholtz resonators for attenuating engine noise. For instance, by including acoustic treatments incorporating honeycomb structures in the nacelle housing, engine inlets and exhaust structures are provided to reduce cabin and environmental noise due to the jet engines. Some acoustic treatments use perforation of one or both of the face sheets to create the acoustic attenuating assembly. A resonance absorption frequency of the treatment structure is determined by cell sizes and hole dimensions, and thicknesses of the face sheets. The cells of the honeycomb core act as resonant chambers in which the noise is transformed into waves of different frequencies and this sound energy is then converted into heat, which dampens the incoming noise. One downside of this approach is that fabrication costs of drilling holes of the face sheet are high and labor intensive, and are limited to large sizes.
Other designs for honeycomb-based acoustic damping structures involve an assembly of a double layer of honeycomb core with a septum bonded between the two honeycomb sections. Facing sheets are applied to both of the external surfaces of this assembly with an adhesive. Construction of this assembly requires multiple adhesive bond lines that reduce the mechanical strength of the overall panel. Alignment of the individual cells can also be difficult between adjacent sections. Furthermore, heat transfer is often degraded because of the insulating adhesive bonding interfaces and the misalignment of the metal cells.
In still other designs, septum caps are placed at different depths within the lumen of the honeycomb cells. Generally complex machining and assembly techniques are required for these designs, and thus add to the expense and labor of this approach. There are other numerous examples in which cores are modified, and even crushed in some cases, to create sound barriers.
Further, externally formed foam plugs have been used to fill the cavities, but are expensive to insert and lack depth control.
Existing fused porogen and/or particulate leaching processes have been utilized to fabricate polymer foams used in several different applications, particularly relating to bio-scaffolds. These processes can involve a particle fusion step, followed by application of a biodegradable polymer and particulate leaching to reveal a polymeric foam structure. These types of structures have not been utilized for acoustic cores. Additionally, existing types of structures would likely suffer from several drawbacks in application to acoustic damping structures, including ease and control of insertion and septum adhesion to the core, which is desired for acoustic performance. In addition, the requirements for open porosity on both sides of the septum, i.e. continuous flow permeability, distinguish the needs of acoustic septa fabricated using the particle fusion scaffold technique from previous applications of this technique. Furthermore, existing materials sought to fabricate polymeric foams from fused porogen and/or particulate leaching processes are generally unsuitable for aerospace and automobile applications, e.g. due to the temperature, vibration, chemical, and strain environments.
What is needed is an acoustic structure and fabrication process that is a scalable process and does not require multiple insertion steps for each individual cell of the structure, to enable a lower cost and less labor intensive process, that has greater flexibility in terms of cell size and porosity of the structure.